The subject invention relates to a rotating heat exchanger in the form of a cylinder having double jackets, comprising a heat exchanger section for passage-through of the fluid serving as the heat exchanging medium in the space between the cylindrical outer and inner jackets.
Prior-art heat exchangers of the type defined above are constructed in such a manner that the entire cylinder rotates both when it is supported by a through shaft or--more frequently--by a pair of stub axles secured to the cylinder ends. This means that shafts and axle bearings must be dimensioned on the basis of the total weight of the cylinder. Cylinders of this kind are used for instance in machines for production and drying of paper and in machines of this and similar nature the cylinders may have a length of up to 7 meters, a diameter of 1.5 meters and a weight of approximately 5 tons. The considerable weight presents additional disadvantages which are more important than the one already mentioned, i.e. the necessity to use large-size axle bearings.
One such additional disadvantage is due to the necessity of supplying and leading off the heat-exchanging medium via rotating seals. The latter must be replaced or repaired comparatively frequently due to considerable damages caused to them from wear, as a rule of impurities present in the heat-exchanging medium which in the case of papermaking machines generally consists of water or steam. Replacements mean operational standstills and in addition are costly in themselves, since packing boxes designed for rotating axles are expensive.
A third disadvantage connected with the considerable weight of the cylinder is the following one. The external face of the outer jacket must have a very smooth surface finish, particularly when the cylinder is used in calender rolling mills and in similar applications. When used in mills of this kind also comparatively minor scratches in the surface might make the cylinder unfit for use. When damages like these occur, the cylinder must be lifted off the machine with the aid of an overhead crane and the entire cylinder unit be transported to a workshop to be repaired. While the cylinder unit is being repaired a complete spare cylinder unit must be used.
A further disadvantage connected with the great cylinder weight is the correspondingly great inertial mass, which often makes it impossible to use the paper web or equivalent means to drive the cylinders. Instead, the latter must be driven by motor and the drive motors require complicated synchronization mechanisms which are sensitive to disturbances and which usually are thyristor-controlled.
The invention is based on the realization that it is possible to eliminate the above-mentioned disadvantages by constructing the cylinder in such a manner that only its outer jacket rotates while the inner jacket and the rest of the cylinder unit components are stationary. By constructing the cylinder in this manner several important advantages are obtained. Firstly, reduction of the mass of the movable components is considerable, allowing use of smaller and less expensive bearings. Secondly, elimination of the rotating sealing members, because the cylinder comprises stationary parts through which the medium may be supplied and removed. Thirdly, when damages are made to the outer jacket the latter may be conveniently and readily dismantled from the cylinder, while all other components remain in position, which drastically reduces the handling and actual repair costs. Fourthly, the rotating mass is so small that as a rule the web itself will be capable of driving the cylinder jacker, whereby the need for drive motors for several cylinders and synchronizing means therefor become superfluous.
The arrangement in accordance with the invention gives additional advantages which will be described in the following. However, one should be mentioned already at this point. Because the heat-exchanging medium passes through a space inside the cylinder, one of the large delimiting walls of which, the inner jacket, is stationary while the other large delimiting wall, the outer jacket, is rotating a relative movement is generated in the peripheral direction between the medium and the outer jacket. The practical consequence of this phenomenon is that the heat exchange no longer is effected exclusively by conduction but is supplemented to a large extent by convection.
A pronounced turbulent flow of the heat-exchanging medium is generated, resulting in excellent exchange of heat between the medium and the outer jacket.